1. Field of the Invention
The present invention generally relates to environmental test systems, and more particularly to an improved controller for controlling the environmental conditions within a test chamber of an environmental test apparatus.
2. Discussion of the Related Art
As is known, many applications for PC boards involve hostile operating environments, particularly including temperature extremes and sometimes, rapid excursions between such extremes. The operating environment may also include vibration which must be withstood by the PC boards. Unless recognized in board construction and testing, such environments can cause premature failure of the board per se and/or of the components mounted thereon.
To help assure that PC boards provide the requisite degree of reliability in such applications, board manufacturers often subject them to temperature tests (often referred to as "stress tests" or "stress screening") by placing them in an environmental test chamber capable of producing rapid and extreme changes in temperature. For example, such a chamber might provide a temperature change of from 40.degree. F. to over 200.degree. F. in about 30 minutes. The stress test may also include vibrating the PC boards being subjected to such temperature extremes. Stress screening pulls out PC boards that exhibit what is often called "infant mortality"--i.e., boards which fail prematurely under the rigors of such screening.
In addition to temperature and stress testing, those who make PC boards have sometimes been required to impose yet an additional form of stress--that of changing humidity. An example of an application for PC boards subjected to the additional rigors of humidity testing is aircraft electronic systems. Both commercial and military aircraft can be exposed to both very dry ambient air and very humid ambient air in a relatively short time.
Until the advent of the invention, those who test PC boards by stress screening had three choices, none of which were particularly attractive from an economic standpoint. One choice was to anticipate the need for humidity testing by purchasing (at added cost) an environmental test chamber configured with a humidity-changing capability. But relatively few PC boards are required to be tested under controlled humidity conditions since the humidity-related components may be underutilized or not used at all.
Accordingly, many environmental testing devices of the prior art define either a temperature testing chamber or a humidity testing chamber. Indeed, U.S. Pat. No. 5,631,429 (the '429 patent) characterizes a shortcoming of prior art testing systems as requiring separate testing apparatus for performing temperature testing and humidity testing. Accordingly, the '429 patent discloses an environmental testing apparatus and method that allows and performs both types of testing in a single apparatus. However, as contemplated therein, the temperature testing and humidity testing are to be performed at different times, often spaced apart by several months (see col. 2, line 57). Indeed, a separate "humidity module" must be installed in that device when humidity testing is desired.
One shortcoming of prior art systems relates to condensation build-up. Indeed, the '429 patent notes the significance of a drain conduit that is used to remove any condensation forming in the chamber, during temperature testing (the conduit has a different purpose during humidity testing). High levels of condensation within a test chamber can result in increased incidence of component failure, particularly when testing sensitive electronic equipment, such as integrated circuit components.
Accordingly, what is desired is an improved environmental test system that accommodates simultaneous temperature and humidity testing, and which provides improved condensation control.